This invention relates to the transmission of baseband video signals.
A need exists to bring baseband video signals to the desk in office environments, for example, to provide security monitoring, factory production monitoring and video conferencing. Such information needs usually require the installation of shielded wiring or optical fiber cables. It would be more economical to utilize wiring which is already available within the physical structure, i.e., unshielded twisted pair cables used for transmission of analog voice signals in the telephone system. Such a video transmission is offered, for example, in AT&T's Systimax.RTM. Premises Distribution System. (See, e.g.,Systimax.RTM. Premises Distribution System Components Guide, AT&T Doc. No. 3726C (Dec. 1990).)
Recent proposals have suggested transmitting composite baseband video signals over a twisted wire pair using active components to compensate for phase delays. (See, e.g., U.S. Pat. No. 5,130,793 issued to Bordry et al. and U.S. Pat. No. 5,172,413 issued to Bradley et al.)
It would appear to be more economical to couple video signals onto twisted wire pairs using "passive" rather than "active" components as described, for example, in U.S. patent application of Georger and Rutkowski, Ser. No. 07/943,644, filed Sept. 11, 1992, and U.S. patent application of Georger, Ser. No. 08/037,944, filed Mar. 26, 1993.
Transmission of more than one non-synchronized video channel over corresponding twisted wire pairs which share the same cable sheath presents particular problems in the amount of induced electrostatic field crosstalk which can develop between adjacent pairs due to the unbalanced voltage condition established by the video signals. A passive coupling approach would necessitate use of a transformer to balance the signals. However, such a transformer would also be required to pass a DC signal which is present at the low end of the video bandwidth.
U.S. Pat. No. 3,731,237 issued to Beurrier suggests use of an autotransformer including a trifilar winding for transmission of broadband signals. However, the transformer cannot pass DC signals.
U.S. Pat. No. 4,885,747 issued to Foglia uses transformers with three windings to couple a broadband video signal and baseband data signal onto shielded twisted pair cables. The baseband portion of the design utilizes a common-mode choke which by its nature does not balance the voltage portion of the signal. Only the current portion is balanced thus reducing the amount of crosstalk due to the magnetostatic field but not the electrostatic field.
U.S. Pat. No. 5,010,399 issued to Goodman et al. uses a transformer with three windings to modulate a baseband video signal so it can share the same twisted wire pair as the voice signals.
It does not appear, however, that the prior an has taught how to passively couple non-synchronized baseband video signals onto separate twisted wire pairs which share a common cable sheath.